It is a well established fact that to reinforce and stabilize underground rock formations, such as a coal mine roof, an underground tunnel, or any subterranean structure, the application of roof bolts, inserted into boreholes drilled into the rock formation, is the recommended standard practice and often is required by Federal law. The bolts fall into three generalized categories i.e. 1) passive support, 2) active support, and 3) frictional supports.
Passive supports involve installing the grout or resin anchoring systems into the borehole ahead of the reinforcing rod. This can be accomplished by inserting the material into cartridges that are ruptured as the end of the rod is forced and usually rotated at the same time through the cartridge. The material fills the annulus along the borehole wall, cures with time, and then uses a mechanical interlock mechanism for anchoring the rod unit. Another option for passive support system is to place the rod into the borehole and pump the pressurized anchoring material into the borehole along the rod or through the center of the rod. Passive support systems are loaded by allowing the immediate roof to deflect downward, thus placing the rod into tension.
Active support systems can be placed into the borehole with a threaded end that accommodates a mechanical expansion shell. Rotation of the bolt advances the calming plug downward relative to the shell to expand the fingers on the plug into a gripping engagement along the borehole wall. By continuing to rotate the bolt, higher levels of tension are generated along the roof bolt axis. Another method for creating active support systems was to install the rod in a similar fashion to passive supports. When the anchoring material has cured the bolt was placed into tension by turning a nut on the bottom end the desired level of tension has been obtained. The levels of tension are usually determined with a torque wrench.
Frictional supports are placed into a borehole by applying force at the end of the bolt to deform the unit as it is pushed into the hole or by expanding the bolt with compressed water after it is placed into the borehole (U.S. Pat. No. 4,511,289). Both of these systems rely on the friction generated between the bolt and the borehole wall to resist movement as load is applied via the bearing plate.
U.S. Pat. Nos. 3,108,443; 3,892,101; 3,940,941; 3,979,918; 4,051,683; 4,127,000; 4,129,007; describe systems that use a grout or resin to anchor a roof bolt into a rock formation. U.S. Pat. Nos. 3,925,996 and 4,216,180 describe multi-component resin systems in which the resin mixture cures and hardens within seconds after a thorough mixing.
U.S. Pat. Nos. 3,877,235; 4,051,683; 4,023,373 ; and 4,275,975 disclose chemically anchored roof bolt systems that include an anchor portion which is inserted into the borehole behind the resin cartridges and a lower portion which is connected to the anchor portion. With these types of support systems, once the resin has been adequately mixed and cured to adhesively secure the anchor portion in the borehole, torque is applied below the anchor portion by rotating the bolt relative to the anchor. This draws the roof plate on the end of the bolt in relative to the rock formation. In this manner the bolt is put into tension.
Recent U.S. Pat. Nos. 4,413,930 and 4,419,805 disclose methods and apparatus for combining resin bonding characteristics with mechanical anchoring of the bolt in the rock horizon. With these devices a single bolt with a mechanical anchor positioned along the upper threaded end of the bolt is inserted into a borehole behind a tube or tubes of resin or cement cartridges. The roof bearing plate is carried on the opposite end of the rock bolt to accept and apply loads against the rock formation surrounding the open end of the borehole. The cartridge system is ruptured by simultaneously pushing and rotating the bolt to release and mix the cartridge components, normally resin, a stop device, or other means associated with traditional expansion anchor designs, restrains expansion of the shell when the bolt is rotated in the selected direction to mix the resin components. Rotation of the bolt continues until the resin has been thoroughly mixed according to the manufacturers recommendations. As the resin mixture begins to harden, the shell expands into the engagement position with the borehole wall and further rotation of the bolt exerts a tension component into the bolt. The tension component extends from the base of the anchor to the head of the bolt.
Rock strata and rock formations above underground openings can include homogenous materials as well as bedded formations. Bedded formations can include a variety of sedimentary materials such as shale, mudstone, coal, claystone, and other types of rock formation. These layers can vary in thickness and occur in random orders that deviate from horizontal bedding planes. While certain layers, such as sandstone, can have high strength characteristics, they may also occur in very thin beds that would prevent proper support utilizing only expansion anchor support systems. With this type of roof it becomes necessary, in the past, to drill boreholes through the strata until a stable horizon could be located to permit the proper anchorage of the bolt expansion shell. This may require inordinately long holes or in several no stable horizon could be located. Even if a stable horizon can be located, the anchor placed into tension by subsequently tightening the bolt can fail because of anchor slippage, relaxation, and a deterioration of the material underneath the bearing plate. This type of unit with the subsequent post-installation behavior, without being placed into and maintaining tension, would be analogous to installing no support and create hazardous conditions.
In an effort to maintain a larger contact area between the expanded shell member with soft rock strata, special multiple anchors positioned in tandem on a bolt have been developed and disclosed in U.S. Pat. No. 3,469,407.
U.S. Pat. No. 2,525,198 discloses an anchor bolt that includes an upper threaded bolt and a tubular member of a preselected length. A lower expansion anchor shell assembly is positioned on the lower threaded bolt. With this arrangement the contact area between the bolt and the borehole wall is expanded. Bolt tension in this unit is still generated using a mechanical method. U.S. Pat. No. 3,303,736 discloses an expansion shell assembly adapted for positioning anywhere on the threaded potion of the bolt.
While it has been suggested by the prior art, devices that utilize mechanical anchors or a combination of mechanical and chemical anchors to secure a bolt in a borehole, to overcome problems associated with obtaining adequate anchorage to place a bolt in tension, are severely limited. These limitations include: the development of adequate anchorage to accept and maintain high tension loads, methods to minimize and eliminate bolt load tension installation losses due to the friction between the bearing plate and the head of the bolt, methods to minimize or eliminate friction between the threaded portion of the bolt and the expansion portions, and a method to install and maintain predetermined tension loads using only the properties of the bolt installation procedure. These factors all influence the final installation tension, which greatly affects the overall stability of the subterranean excavation.